PROJECT SUMMARY Current treatments for hemophilia A consist of frequent FVIII infusions to maintain hemostasis. Replacement therapy is lifelong, expensive, unavailable for ~75% of the world's hemophiliacs, and >30% of patients develop inhibitors to Factor VIII (FVIII). There is thus a critical need to develop affordable HA therapies with longer- lasting benefit or permanent cure. Clinical trials directly infusing AAV vectors for in vivo FVIII gene delivery have yielded promising results in adults, but pre-existing neutralizing antibodies (NAbs) to AAV are present in up to 50% of the population and inhibit target cell transduction when AAV is delivered systemically. As a result, clinical trials using AAV can currently only enroll subjects with no- or low-titer pre-existing AAV NAbs. Prenatal treatment (PNTx) for hemophilia A would circumvent the immune barriers present in adult patients, and could induce lifelong tolerance to FVIII, and thereby eliminate the risk of FVIII inhibitor formation, the most feared problem in treatment/management of hemophilia A. Although direct injection of viral vectors into prenatal recipients would likely be successful, numerous safety concerns need to be addressed before this approach can enter the clinic. A cell and gene therapy-based approach, in which cells are transduced in vitro, would enable multiple safeguards to be implemented in production, e.g., copy number and integration site analyses, that are not possible with direct vector injection. Such an approach would also eliminate the possibility of off-target gene-modification. Using a large animal model of PNTx, we showed that an off-the-shelf therapy comprised of human placental cells (PLCs) transduced with a lentiviral vector (LV) encoding a bioengineered, codon-optimized FVIII transgene, designated mcoET3, resulted in plasma FVIII activity levels that exceeded that of control non-transplanted animals by over 20%, and that these levels were maintained for at least 1 year after birth. Based on these results, a Pre-IND meeting with the FDA was held to discuss the path to a clinical trial. It was agreed that while it was vital to consider safety of the product to safeguard the patient, it was also imperative to protect the mother, particularly with respect to possible exposure to the gene- modified cells and/or gene product. To directly address this FDA guidance, we herein propose to: Aim 1: Manufacture, characterize, and authenticate, in vitro, a gene-modified PLC-based hemophilia A therapeutic product under cGMP conditions; and Aim 2: Confirm the therapeutic potential and safety of mcoET3- expressing human PLCs in vivo, in mice and in a large animal model of PNTx, determine whether there is transfer of the product across the placenta to the mother following PNTx, and if so, define the immune consequences of such transfer. At the completion of this project, we expect to have demonstrated the innocuity of the PNTx approach, and to have developed a safe and effective PLC-based hemophilia A therapeutic that could be used in a PNTx Phase-I clinical trial.